The invention relates generally to an acoustic detection system, and, more particularly, to a system which is sensitive to sounds outside the frequency range of human hearing to enable a user to enhance his hearing capability.
It is often required in police surveillance and military operations to passively detect the movement of personnel or machinery at extended distances. In many instances, vision is obscured due to darkness, low visibility, obstacles, or dense vegetation. Reception of sounds produced by personnel and machinery may thus be the only method of detecting their presence or movement. However, reception of sound waves solely in frequencies audible to human beings may not provide adequate detection capability.
Sounds outside the frequency range of human beings may, however, exhibit significantly different propagation characteristics. For example, ultrasonic (higher than audible) sound waves produced by movement of personnel, whispering, the impact of metal parts, and sharp explosions, such as from firearms, are more directional than audible sound waves. Infrasonic (lower than audible) sound waves produced by heavy machinery, large vehicles, and helicopter blade chop, on the other hand, may propagate over greater distances. It is therefore desirable to provide the capability for detection, identification, and classification of noise sources for sounds such as those described above in the ultrasonic and infrasonic ranges. Similar requirements also exist in many fields of scientific research, where the events of interest include animal and insect sounds produced in the ultrasonic range and meteorological and seismic events which produce sounds in the infrasonic range.
Events of interest may well produce a variety of sounds over a wide range of frequencies extending from below the lower frequency limit of human hearing to well above the upper frequency range thereof. Initial indication of events of interest may occur in one frequency range, whereas sounds confirming the existence of such activities may be produced in other ranges. It is therefore desirable to provide acoustic detection apparatus which can detect sound waves over a wide range of frequencies both inside and outside the frequency range of human hearing. Equipment which can detect ultrasonic or infrasonic waves has up to this time been large, heavy, and expensive, and was thus more suited to the laboratory than to the field. Accordingly, it is an objective of the present invention to provide acoustic detection apparatus which is low in cost, rugged, and light in weight so as to be suitable for portable operation in the field